This invention relates to deck drainage in offshore oil platforms and the recovery of oil from an oil containing fluid mixture collected by the deck drains. In particular, this invention relates to such deck drainage and oil recovery at an oil and/or gas processing facility located adjacent to a body of water, and to the disposal of the relatively oil-free fluid into the body of water.
It is conventional in oil and/or gas drilling and production to collect rainwater and miscellaneous fluids such as oils and greases for example from the decks of an offshore platform. Typically, a plurality of drains are arranged throughout one or more decks of the offshore platform, especially on portions of the decks which are open and therefore exposed to the weather. Since the rainwater washes any spilled oil or grease off of the deck and into the drains, the rainwater cannot be passed directly into the body of water beneath the platform. Instead, the collected rainwater must be treated so as to separate the oil from the water until the percentage of oil in the water reaches an acceptable level. Typically, (as the governing country's ordinances permit) as little as 30 ppm of oil is permitted in the water to be returned to the body of water beneath the platform.
Sometimes, the recovery of the oil present in the collected water is economically worthwhile because of the value of the recovered oil. Both for pollution prevention and for economical reasons, it has been conventional to supply collected rainwater and oils from the decks of an offshore platform to a "Skim Pile" (hereafter skim pile) or elongate separator conduit of Engineering Specialties, Inc. Covington, La. such as is described in U.S. Pat. No. 3,893,918.
In U.S. Pat. No. 3,893,918, issued July 8, 1975, there are disclosed skim piles or elongate separator conduits that are useful and efficient in such recovery and disposal operations. The length of the conduit and the operating procedures are such that the rising velocity of the oil in the mixture results (during an intermittent flow procedure) in the establishment of a predictable upper oil layer in the conduit.
One of the skim piles disclosed in that patent operates while in continuous communication with the body of water. As such, the total fluid level in the skim pile is responsive to tidal changes in the mean level of the water body. Provision is made for withdrawing oil from the oil layer on top of the total fluid column in response to sensing of the level of the interface between such oil layer and the remaining fluid column therebelow. In that fashion, one seeks to insure that only an oil rich mixture is withdrawn from the skim pile. Although that technique has proved quite acceptable, some difficulties have been encountered especially in situations where unpredictably large or erratic total fluid level changes are produced in response to changes in the mean level of the body of water.
U.S. Pat. No. 3,893,918 also discloses a form of skim pile where the level of total fluid in the skim pile is basically isolated from changes in the mean level of the water body through the use of a valve. Problems associated with changes in that mean water level are thereby avoided. Such skim piles are normally employed only in shallow water situations where the underwater extent of the skim pile, if it were open at the bottom, would not normally be great enough to allow for good separation of oil (as governed by the sizing charts). This is because the terminal rising velocity of the oil would not sufficiently overcome the exit velocity of downward flow in the conduit (during the periods of intermittent flow). Although normally closed and valved skim piles could be employed in deeper water situations, operational and maintenance difficulties could become prominent by reason of the need to control a valve positioned at great depth.
In a conventionally known arrangement for collecting rainwater and oil containing liquids from one or more decks of an offshore platform facility, a liquid seal strainer is provided immediately upstream of the inlet of the skim pile with all of the deck drains supplying the collected fluids to the liquid seal strainer and then immediately into the skim pile.
Such an arrangement is objectionable, however, because the deck drains of different decks may be in communication with one another. Harmful gases which oftentimes accompany the collected rainwater and oil mixture from one deck could then be transferred inadvertently to another deck.
In the known deck drain assemblies, no provision was made for removing the harmful gases from the drainage system but instead, both the liquid and gaseous components of the collected fluid were supplied to the skim pile. Since the skim pile is usually located lower than the deck drains, the gas must therefor be forced downwardly against gravity with the result that the gas has a tendency to remain trapped in the drainage system in an undesirable manner.
Still further, since such gases which may be present in the collected rainwater and oil are very often highly flammable, the possibility of an explosion is ever present. Even though the skim pile is provided with a vent to atmosphere, the danger resulting from the harmful and flammable gases is increased since the gases are detained within the drainage system until they reach the skim pile and since the gases are passed perhaps from the top deck of the platform down to the boat deck of the platform where an explosion of the gas could likely result in greater damage both to the platform and to the crew.